The invention relates to a method for steering a vehicle and to a vehicle for carrying out the method.
The invention relates in particular to a control of a steering system for passenger cars with a motor for providing steering assistance or for steering alone (steer-by-wire). The motor is partially or entirely used to perform the transverse guidance of the vehicle by means of a vehicle-specific control process. So that the driver is able at any time to take over the steering function again completely, automatic transverse guidance of the vehicle is deactivated by means of closed-loop steering control after a steering intervention by the driver has been detected.
In known steering systems with an electronic closed-loop steering control unit which sets a predefined travel trajectory by means of automatic steering interventions, the automatic steering intervention is usually also ended at the transition of the automatic closed-loop steering control from the switched-on state into the switched-off state. It follows from this that the steering torque which is applied to the steering system by the automatic steering intervention is initially reduced in a jolting fashion until the steering torque which is present at the steering system allows the steering torque in the steering system to increase again solely on the basis of the manual torque applied to the steering wheel by the driver. This jolting sudden decrease in the steering torque gives rise to a perceptible reduction in the curvature of the travel trajectory which is set by the closed-loop steering control unit and which does not correspond, with regards to value and direction, to the change in curvature which is expected by the driver and indicated by his manual steering intervention.
An object of the present invention is to provide a method for steering a vehicle which permits safe operation of the vehicle. In particular, an object of the invention is to prevent, in the case of manual steering intervention by the driver during an automatic steering intervention by a switched-on closed-loop steering control, a sudden decrease in the curvature of the previously automatically set travel trajectory.
The object is achieved in accordance with embodiments of the invention.
According to the invention, a method is provided for steering a vehicle. The forces for steering the wheels of the vehicle are generated by a steering torque which is referred to below as ML. The steering torque ML is applied by way of a mechanical steering system and/or a steering actuator.
In the case of a conventional steering system, that is to say a mechanical connection between the steering wheel and the steered wheels, the steering actuator generates an additional torque which is directed in the opposite direction to or the same direction as the manual torque applied by the driver. In this context, the steering actuator is comparable to the drive of a power steering system. Given corresponding actuation, the steering actuator can also be used to steer the motor vehicle automatically. This does not require any manual torque.
As an alternative to the conventional steering system, a steer-by-wire system can also be used. In the case of a steer-by-wire system, there is no mechanical connection between the steering wheel and the wheels. The manual torque is merely sensed by means of a sensor and transmitted to the steering actuator.
In the method according to the invention, the vehicle is firstly operated in an automatic phase. In this context, automatic steering of the vehicle is carried out by a control unit calculating an automatic torque MAF. During the automatic phase, the steering actuator is actuated in order to generate precisely this calculated automatic torque M. During the automatic phase, the driver does not apply any manual torque MMF to the steering wheel. The steering torque ML correspondingly corresponds to the automatic torque MAF.
According to the invention, a driver intervention is sensed during the automatic phase. Furthermore, sensing of the applied manual torque MMF takes place. The sensing of the driver's intervention is advantageously carried out by sensing the manual torque M. Alternatively or additionally, the driver's intervention can also be sensed by further sensors or cameras.
As soon as the driver intervenes in the automatic steering system, the so-called intervention phase begins. In this context, the calculated automatic torque MAF is maintained as a steering torque ML. This is done either by ignoring the manual torque MMF in the case of a steer-by-wire system or by compensating the manual torque MMF in the case of a conventional steering system. In this context, the automatic torque MAF drops or rises in order to compensate the manual torque MMF. In the intervention phase, the activation of the steering wheel by the driver therefore has no effect whatsoever on the transverse steering of the vehicle.
The intervention phase lasts until the manual torque MMF has exceeded a defined threshold torque MS. According to the invention, the threshold torque MS is greater than zero here.
After the intervention phase, the transfer phase follows. In the transfer phase the automatic torque MAF is reduced. The steering torque ML and therefore the actual transverse guidance of the vehicle therefore correspond to the sum of the manual torque MMF and the automatic torque MAF.
Through the need to suppress the defined threshold torque MS, the driver is provided with better haptic feedback that the system for automatic closed-loop steering control is active. A subsequent drop in the automatic torque MAF provides haptic feedback that the system is switched off and the driver is responsible for the steering function again.
The setting of a constant steering torque ML until the defined threshold torque MS is exceeded in the intervention phase ensures that the application of the threshold torque MS for switching off the automatic closed-loop steering control does not have any undesired effects on the automatic closed-loop steering control which is still active up until then.
There is preferably provision that during the transfer phase the automatic torque is reduced to zero in a continuous profile. This continuous reduction in the automatic torque MAF takes place over a time period which is longer than zero. The controlled reduction in the automatic closed-loop steering control permits the driver to take over the steering function without in the process experiencing an undesired change in the travel trajectory as a result of a sudden drop in the steering torque ML.
There is preferably provision that in the intervention phase the automatic torque MAF is maintained as a steering torque ML both in the case of a manual torque MMF which is directed in the same direction as the automatic torque MAF and in the case of a manual torque MMF which is directed counter to the automatic torque MAF. This means that even if the driver would like to steer counter to the automatic steering system, he must initially overcome the threshold torque MS until the automatic torque MAF is reduced.
There is preferably provision that the threshold torque MS decreases over time during the intervention phase. In this context, the threshold torque MS particularly preferably decreases digressively. By defining a chronologically variable value for the threshold torque MS, it is possible to compensate unintended strong steering interventions without removing control of the vehicle from the driver. The system continues to remain active in the case of brief impacts.
In this context there is provision, in particular, that at the start of the intervention phase the threshold torque MS has an initial value and drops to a constant minimum value. The minimum value is greater than zero here. The initial value is advantageously at least 3 times, preferably at least 4 times, the minimum value. The minimum value ensures that even permanent unintended steering interventions by the driver do not cause the automatic system to switch off.
The invention also comprises a vehicle. The vehicle is, in particular, a two-track road vehicle. The vehicle is designed to carry out the described method. In particular, the vehicle comprises for this purpose a steering actuator which can be correspondingly actuated.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.